Many different methods are known in the art which can be used for the detection of molecules. In particular, microarray technology has evolved into a powerful tool for the detection of many different types of molecule e.g. DNA, RNA, protein, antibodies and chemical compounds. Microarrays have been developed to study gene expression, to identify gene mutations, aneuploidy and polymorphisms and to identify protein:protein interactions, to name only a few uses. This technology has proven to be an important technique, particularly for research and for diagnostics.
Microarray chips generally consist of a collection of microscopic molecule spots attached to a solid surface such as glass, silicon or a nylon membrane. Target molecules in a sample can be assessed by determining their binding pattern to molecules on a particular microarray. Target molecules are usually fluorescently labelled and a microarray can therefore be read and a target molecule detected by determining the presence (and level) of fluorescence at each molecule spot on the microarray, which corresponds to the presence (and amount) of target molecule present in a sample. The use of fluorescent labels however requires the use of fluorescence scanners which although result in accurate and sensitive methods, are expensive. The requirement for expensive scanners has prevented the use of microarrays as a routine tool in research and in diagnostics.
Additionally, it is not usually possible to detect single molecules using fluorescent labelling and microarray technology. Particularly in the case of nucleic acid detection it is usually necessary to carry out an amplification step prior to applying a sample to a microarray. Fluorescent scanners/detectors are not usually sensitive enough to detect the presence of a single fluorescent label and hence the presence of a single molecule.
Mallard et al (Biosensors and Bioelectronics, 20, 1813-1820, 2005) developed a CMOS photodetector array as a solid support for a DNA chip where detection was by means of chemiluminescence. Although this system did not require the use of an expensive detector or fluorescent scanner, it again was not sensitive enough to enable the detection of a single molecule. Tokuda et al (Proceedings of the 2005 IEEE, Engineering in Medicine and Biology 27th Annual Conference, Shanghai, China, Sep. 1-4 2005, 7269-7272) further developed a CMOS image sensor for bioimaging applications in which fluorescence imaging was used.
Chan et al (US2006/0084069) developed an integrated circuit optical detector as a solid support for an array where the presence of a target was detected by receiving and sensing optical signals from a nanoparticle label. However, although this method did not require the use of an expensive scanner or detector, the optical detection of very small nanoparticles was not sensitive enough to detect single molecules.
Thus there exists a need for an array molecule detection system which is capable of detecting single molecules and which does not rely on the use of expensive scanners or detectors.